Why are automotive engine force output specified in terms of torque [Nm] and not forward thrust [N]?

Question

For instance, an example catalogue extract for a motorcycle reads-

Bore / stroke : 102,5 mm x 76 mm. Capacity : 1,254 ccm. Rated output : 100 kW (136 PS) at 7,750 U/min. Max. torque : 143 Nm at 6,250 U/min. Compression ratio : 12,5 : 1.

From a user's point of view, surely they are more interested in how much forward thrust the vehicle can provide, as opposed to the torque at the rear wheel (which is also implementation dependent). Thrust is also more easily relatable to other parameters of interest, such as acceleration (assuming a known mass).

Perhaps there is an argument that thrust is dependent on further variables such as the effective rear wheel outer diameter (tyre pressure, tread etc.), road conditions (amount of slip), so only a reference figure can be given. However, I would argue back that torque by itself gives less information in terms of user experience.

From a quick check, aircraft engines are specified in terms of thrust, and not torque. So why not for bikes and cars? Interested to hear different points of view.

Example specs from BMW R1250GS product page- https://www.bmw-motorrad.co.uk/en/models/adventure/r1250gs/technicaldata.html#/section-technical-data

Answer 1

I'm not a 100% sure about the correctness of this answer, (please comment if you see errors)

First of all, the torque number specified, just says so much. The number is actually just the max value of a curve (because the torque is dependent of the engine speed). The torque curve is the interesting thing.

This means that if we compare two cars with the same weight, it's not certain that the one with highest max torque and max horse power will win in a dragrace. And even if the two cars had the exact same torque and hp numbers, it's still not certain that the one with the higher numbers would win. Because it's also dependent on the gearbox gearing and final gear (and wheel size).

The following picture shows a torque curve for a car, this curve also includes the gearbox!

The colors except the red shows the torque for each gear, and the red one just just the combination.

Now for the thrust. Implicitly the thrust curve is given by the torque curve combined with wheel size and final gearing. If you do some multiplications of these numbers you will (I think) get the thrust curve. As you can see the thrust curve will give you a peak in the low speeds (around 30-35 mph in this case). So now if you provide the thrust curve it says some about the acceleration, maybe more understandable than just the torque figure.

However you'd have to take into account the weight of the car, the wind- and rolling resistance etc. etc. to get a good view of the acceleration (which I assume is the interesting thing). And in that case, couldn't you just have a acceleration curve?

Also, it's more likely than not that the manufacturer would think it's simpler just to provide a number, and not the full curve. This would probably be the max number, and the question is how interesting the max thrust actually is since it only occurs once, and then drastically falls. So in that case isn't is just better with the 0-60 numbers as @Kitsunemimi says?

Answer 2

The problem with forwards thrust is that it's a property of the entire vehicle (with a certain transmission, certain final drive ratio and certain drive size), not a property of the engine. Also it varies depending in the gear you're using.

There's one parameter that's independent of all of this. It's power, specified in horsepower (note there are two different incompatible definitions of "horsepower") or kilowatts. That's a physical restriction of the engine you can't overcome. If an engine produces 100 kilowatts, you can't ever get 101 kilowatts out of it, but if an engine produces 200 Newton meters at the output shaft you can easily get 2400 Newton meters out of it at the wheels -- just select the first gear and floor the gas pedal.

However, the problem with maximum engine power in kilowatts is that it's almost always produced in a rotation rate that makes the engine very noisy. If you want to accelerate quickly, you can shift to a gear providing higher rotation rate of the engine, but the shift is slow and bad for the user experience of the car. Also if you don't want to annoy other road users with the noise, you might want to not do that shift.

Therefore, you may want another kind of number that describes how well the engine provides rotational thrust (that transmission and tires convert to forwards thrust) at the current rotation rate, something that is as independent of the rotation rate as it can be. That's torque. Usually the torque curve is very flat over the useful operating region of an engine.

The problem with engine torque is that transmission and final drive convert it to wheel torque and you need to do that conversion to get wheel torque. Also wheel size is needed to convert that to forwards thrust in Newtons. But, in some cases, if you want to know how well you can accelerate in a reasonable gear without doing a gearshift, torque is the figure you're looking for.

Theoretically, in a car with 6-speed manual transmission gearbox, you could specify six numbers for forwards thrust in every gear. Or you could specify only one number, engine torque. Usually engine torque is chosen (along with engine power) because it's a single number that represents the characteristics of the engine. Also in two different vehicles with same engine but different transmissions, final drive ratio and different tire sizes, the torque of the engine stays the same but forwards thrust doesn't. Also what gear you're in is usually dictated by the desire to avoid engine noise and the speed of the vehicle, so you might not even find useful how much forwards thrust you can have in the first gear if you're on a motorway.

When comparing apples to oranges, for example when comparing Wärtsilä-Sulzer RTA96-C that has maximal torque at 102 RPM, with a car gasoline engine that has maximal torque at 4200 RPM, then it's unfair to compare torque (well it would be unfair anyway since the RTA96-C wins always!). Then you want to compare power. However, for two engines with similar characteristics, such as two internal combustion engines that are intended for cars and not for ships, then torque comparisons may be useful and in some cases even better than power comparisons.

Aircraft jet engines produce linear thrust and not rotational thrust. They expel a stream of gas at high speed and that produces linear thurst. Therefore, the figure you're looking for in aircraft is linear thrust and not torque. You can't switch gears in an aircraft!

Answer 3

There's a lot that can be said about this topic, but Solar Mike and Moab pretty much sum up the main reason for it. Torque isn't a metric that was derived for the purpose of describing "user experience", it's a measurement of the amount of force that an engine is capable of producing. Engines spin, therefore they produce torque. Rockets and jets push, therefore they produce thrust (generally). At the end of the day, it's just another number that describes the characteristics of a vehicle - as with many such numbers, you need to understand how to interpret it for it to be useful to you. The number you're probably more interested in is the 0-60 mph time.

Answer 4

I'll add a little to Kitsunemimi's answer (which is totally correct).

Simply, thrust is ONLY a linear force pointing in a single direction.

As stated, everything with ICEs ultimately is culminating in rotational energy (torque) which will lead to thrust (in a sense, but not really). Even where the rubber meets the road, we're not dealing with linear force, but rather many rotational forces that combine, moment after moment, to produce linear motion. (Note: a good way to see this is to observe a drag racing car's back wheels on launch in slow motion. Amazing.)

As you were noticing, all of the other factors of "user experience" are quite variable: road friction, tires, load weight, et al. The one constant that the manufacturer can sell to you is the stock engine output - independant of all these variables - which is just the torque output. Finally, as mentioned throughout, what ultimately determines the actual Nm/s² that the driver experiences will change dramatically depending on gear ratios, tire pressure, wheel size, and even the environmental factors like temp, humidity, pressure, etc.

Even with EVs, they still use torque and HP as ratings because the user can best infer what the experience could be like. They totally bypass the torque curve aspect of the equation, but it's still all about rotational force, rather than linear.